Gas-phase pyrolysis of 1,2-dichloropropane

The gas-phase pyrolysis of 1,2-dichloropropane has been studied at temperatures from 393 to 470°C and initial pressures from 0.2 to 26.0 Torr. In a 1-1. silica reaction vessel of low surface/volume ratio, the products are the four monochloropropenes and hydrogen chloride. Above 440°C the product ratios show only a small dependence on temperature and the relative amounts of the various isomers at 457°C are (expressed as percentage of the total chloropropenes), 3-chloropropene 63.6%, cis-1-chloropropene 22.9 %, trans-1-chloropropene 12.7 %, 2-chloropropene 0.8 %. The overall reaction is closely represented by the pressure change which leads to the overall rate-constant log₁₀k(s^–1)=(13.78 ±0.21)–(54 760 ±440)/4.576 T. Arrhenius parameters have been derived for each of the chloropropene products from the above expression and the product ratios at various temperatures (extrapolated to zero time).

Additions of small amounts of nitric oxide and propene had virtually no effect upon the overall rate. The reaction has also been studied at low temperatures (300–380°C) and in various packed reaction vessels. The product ratios (extrapolated to zero time and corrected for the concurrent homogeneous reaction at the higher temperatures) show increased proportions of 2-chloropropene and cis-1-chloropropene and also formation of propene which can constitute up to 15 mol % of the total organic products. The results are consistent with four parallel unimolecular reactions occurring homogeneously at high temperatures. The heterogeneous reactions are more complicated although similarities are noted with the elimination reactions of other alkyl halides at glass surfaces.